Instead, define it in build/make/Makefile.in.

With this ticket, the MACOSX_DEPLOYMENT_TARGET handling uses a new (unexported) configuration variable defined by sage-env-config; see also #30711.

This cleans up the sage runtime environment - one fewer environment variable.

It is also preparation for #29013.

CC: @jhpalmieri @dimpase @kiwifb

Component: build: configure

Author: Matthias Koeppe

Branch/Commit: 4bfa0ea

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/30724